Submersible well pump assemblies may be employed in oil and gas wells lacking sufficient pressure to lift liquid well fluids. One type employs a rotary pump driven by a down hole motor. The motor is normally electrical, and the pump may be centrifugal. The motor is filled with a dielectric lubricant to lubricate the bearings and assist in cooling the motor.
A seal section or motor protector is coupled between the pump and the motor. The seal section has a flexible member for reducing a difference between hydrostatic pressure in the well and the lubricant pressure in the motor. The flexible member may be an elastomeric bag or it may be a metal bellows. The seal section has a well fluid port that communicates well fluid with one side of the flexible member, normally the outer side. The seal section has a lubricant port that communicates lubricant in the motor with a lubricant chamber, normally the inside of the flexible member. A guide tube may be located within the flexible member surrounding the shaft.
Upper and lower seals seal to the shaft within the seal section. Typically, some well fluid will leak past these seals and eventually enter the lubricant chamber of the flexible member. If the well fluid is able to migrate from the lubricant chamber down into the motor, the life, of the motor will likely be shortened. Encroaching well fluid may particularly be a problem in wells that are inclined. In the past various structures have been provided to cause any well fluid that might enter the lubricant chamber to flow upward, then back downward in a labyrinth or serpentine arrangement. The lubricant is normally lighter than the well fluid, thus the labyrinth makes the journey for encroaching well fluid into the motor more difficult.
While successful, space to provide these labyrinth structures may be inadequate. For example, a metal bellows may have one portion of smaller diameter than other portion. The smaller diameter portion extends into the larger diameter portion. The inner diameter of the smaller diameter portion is often very close or even touching the guide tube. There may not be enough room to include labyrinth pipes in the bellows.